Tactile displays, also referred to as haptic displays, are a promising technology to provide visually-impaired people with efficient and autonomous access to graphical information, such as maps and plots, explored using the sense of fine touch of fingertips. The displays are typically the size of a tablet, and able to refresh the graphical information every few seconds. Electromagnetic (EM) actuation has particularly appealing performance in terms of force, deflection, bandwidth, scaling, integration, robustness and portability. Several EM-based tactile display prototypes have been reported using wire-wound coils to attract or repel small permanent magnets.
One of the most challenging obstacles in a densely-packed matrix of EM actuators with strong magnets is to control the magnetic instabilities on the array due to magnet-magnet interactions. If destabilizing forces are comparable to the actuation forces, taxel (movable pin) displacements can no longer be reliably controlled. Only very few strategies are known to address this issue. One approach is to reduce the magnetic volume. While effective, this restricts the applications to very low-force stimuli because the magnetic interaction scales with magnetic volume. Another approach to reduce magnet-magnet interactions is to use immobile soft-magnetic material housings to enclose and guide the magnetic flux. This option adds significant mass, and is not well suited to light and portable devices. It has also been proposed an alternating up/down magnet orientation on the array to partially cancel the static magnetic field. In a further more effective solution, each permanent magnet is placed in a thin ferromagnetic material thus forming a pot-magnet, where only one surface of the magnet (the surface facing a coil) is unshielded. This allows a dense and compact array of EM actuators with the minimum quantity of soft-magnetic material. However, the problem with this approach is that it is only possible to obtain one stable latching position for the permanent magnet.
Another challenge when designing EM actuators is to keep the magnets stable either in their up or down position. It would be desirable to have two stable magnetic positions, i.e. one stable up position and another stable down position. It is to be noted that the problems identified above are not only present in the field of haptic displays, but these problems are equally faced in any field where EM actuators are used.